Nondestructive eddy current technology is an established technology and various inspection systems exist. Typical systems utilize coupled, multi-turn induction coils often surrounding ferrite cores to intensify induced magnetic field flux. One of the induction coils, the drive coil, is disposed very near the surface of a conductive part undergoing inspection and driven by an alternating current source to create a flux of magnetic field into and below the conductive surface. This flux causes local current to flow in a conductive part. This local current flow induces a mutual magnetic flux of its own. A complementary coil, the sense coil, operates to receive current mutually induced by the resultant flux due to current flow through the conductive part. Coupling between the coils occurs through the conductive part itself. Any flaw or defect in the near surface integrity of the conductive Dart will disrupt the flow of induced current. This disruption can be detected as a change in voltage detected by the sense coil.
A standard eddy current inspection instrument typically utilizes probes made by various manufacturers including: Staveley, Uniwest, Foerster and NDT Product Engineering. Such probes generally have coil elements operating as drive and sense coils which are disposed in close proximity of one another. The probes may differ in their winding arrangement and coil connections. The coils may be wound in the same or opposite directions to accomodate additive or subtractive response signal sensitivity. For example, split core differential probes have coils wound in the same direction while recording head probes have coils wound in opposite directions. Subtractive or "differential" probes generally operate using an impedance mode of detection utilizing a bridge circuit. Differential probes are sensitive to in plane flaw detection making them useful for eddy current testing, although bridge circuit detection can be a disadvantage. One advantage of probes operating in reflection mode is that drive and sense signals on respective coils are more easily separable than they are if a bridge circuit mode of detection is used with a differential sense coil configuration. Typical sense coil configurations include absolute and differential configurations. Absolute configuration utilizes a fixed reference for detection making it useful for calibration. Differentially configured probes utilize a bridge circuit for detection referenced only to ground. Response signals are collected from probes by using manual or mechanical scanning modes. Drive coils can typically be configured as individual coils or in a continuous, serpentine line providing uniform, adjacent, parallel segments driven by an external alternating current source. It is also possible to operate as drive and sense with the same coils using a bridge circuit configuration.
Scanning along the surface of the conductive part being inspected is typically accomplished by moving a probe across a conductive surface to cover all regions of interest. Inspection systems often display a single probe's time trace decomposed into complex sinusoidal components: an in phase component (I), and a quadrature component (Q). Component display is accomplished using an oscilloscope or strip chart recorder. A primary problem in utilizing signal thresholding to determine if a flaw is present somewhere along the scan path involves distinguishing the disruptive flaw signal above background noise. The problem is complicated further as eddy current probes are themselves a source of great variability. Imaging using this approach to measurement collection by scanning with a single probe is time consuming and labor intensive. Furthermore, the image so obtained is spatially blurred by the overwhelming relative size difference between the probe and the flaw to be detected. The use of an inherently spatially correlated measurement array provides simultaneous acquisition of discretely collected data for a plurality of measurements in a single scan. Inspection surface scanning requirements using a spatially correlated measurement array become one dimensional rather than two dimensional as with a single probe. An array spatially correlates one dimension in terms of the other; thus, data collection in one dimension is inherently acquired by scanning in the other. The effective removal of an additional scanning dimension is predicated on providing a spatially correlated measurement array of substantially identical probe elements. One dimensional scanning using such an array is much faster.
Probe sensitivity to small flaw detection is limited by the size of the probe sense coil. The need for miniaturization to reduce this size and improve flaw detection sensitivity has been recognized. However, with conventional fabrication technology the miniaturization required cannot be achieved. In addition to providing decreased probe size relative to that of the flaw, the probe array elements must be substantially identical. Such provisions have not been possible with conventional coil fabrication techniques.
Furthermore, conventional scanning cannot be applied to a wide class of geometrically difficult inspection surfaces. Traditional probe arrays lack a flexible feature that would accomodate scanning such geometries. Scanning with conventional probe arrays lack an alignment feature; thus, alignment becomes time intensive, detracting from useful scan time.